1. Field of the Invention
The present invention relates to a machining control method for controlling axis feed, capable of ensuring stable finish machining.
2. Description of the Related Art
In a known axis feed control method for finish machining, a machining characteristic value, such as the number of electric discharge pulses between a wire and a workpiece or machining voltage, is detected to control an axis feed speed so that the machining characteristic value agrees with a target value, thereby controlling an axis feed along a predetermined path. This control method is intended to obtain a fixed machining groove width after machining is finished. If this method is used in actual finish machining, however, expected machining shapes sometimes may fail to be ensured in the vicinity of corner portions, in particular.
If a workpiece 1 involves a portion that is poor in shape accuracy, as shown in FIG. 1A, a given offset value cannot be secured between a finished surface of the workpiece and a machining path 2. In some cases, therefore, a gap between the wire and the workpiece may be too wide to cause electric discharge, and the surface may fail to be finished. If the gap is too narrow, as shown in FIG. 2A, on the other hand, a hunting or short circuit between the workpiece and the wire electrode may occur. In this case, the surface cannot be finished, and besides, machining must be stopped inevitably.
In a constant-speed feed control method, on the other hand, an axis is fed at a constant speed in the same manner as in the control method described above. Also in this control method, electric discharge cannot be enjoyed at wide-gap portions, and the axis is fed with a short circuit at narrow-gap portions. Thus, uniform surface roughness cannot be secured for an entire machined surface.
A method for improving the surface roughness for finish machining is described in Japanese Patent Application Laid-open No. 11-170118. According to this method, voltage at a gap between a workpiece and a wire electrode is detected, and the wire is relatively moved in a direction substantially perpendicular to a machining path so that the difference between the detected gap voltage and a reference voltage is zero. This method prevents formation of waviness on the machined surface.
Another conventional axis feed control method based on the number of electric discharge pulses is described in Japanese Patent Application Laid-open No. 2002-254250. According to this method, the ratio between the number of electric discharge pulses detected during machining and a reference number of electric discharge pulses is obtained to control a distance of relative movement between a wire and a workpiece that is obtained every given time. Alternatively, a discharge dead time is controlled so that the detected number of pulses is equal to the reference number of pulses. With this method, breaking of wire during machining of corner portions can be prevented, and the machining speed and machining accuracy can be improved.
Another method is described in Japanese Patent Application Laid-open No. 4-30915. According to this method, a period for obtaining an average electric discharge pulse number used for a threshold value is varied depending on cases where the number of electric discharge pulses is increasing and decreasing, individually. The off-time duration of electric discharge pulses is controlled by comparing the obtained threshold value and the detected number of electric discharge pulses, whereby breaking of wire during machining of corner portions can be prevented.
A method of finishing for uniform surface roughness is proposed in Japanese Patent Application Laid-open No. 11-170118. This method is a gap-voltage control-method in which gap voltage between a wire electrode and a workpiece is controlled to be equal to a reference voltage. In order to finish an entire machined surface efficiently and uniformly, the number of electric discharge pulses per unit distance must be controlled to be fixed. The number of electric discharge pulses may be supposed to be fixed if the gap voltage is fixed. Even if the gap voltage is fixed, however, the number of pulses is not always fixed. This is because the machining voltage depends greatly on an electric discharge delay time that elapses from the instant that voltage is applied between the wire and the workpiece until electric discharge starts. In ordinary electric discharge machining, the electric discharge delay time is uneven, so that the machining voltage sometimes may vary despite the fixed number of electric discharge pulses. Thus, satisfactory surface roughness cannot be obtained with use of the control method based on the machining voltage, which is described in Japanese Patent Application Laid-open No. 11-170118.
The foregoing methods described in Japanese Patent Application Laid-open Nos. 2002-254250 and 4-30915 are control methods based on the number of electric discharge pulses. However, these methods are intended to control the feed speed, discharge dead time, etc., and more particularly, to prevent breaking of wire and the like. It is hard for the conventional methods to ensure uniform machined surface roughness in machining workpieces of complicated shapes involving low-accuracy shapes.